This invention relates to a method of preparing a resin coated foundry sand for forming molds and cores for use in a sand mold casting process by using an unsaturated polyester as the coating material.
In conventional sand mold casting processes, molds and cores are usually formed of a resin coated sand prepared by using a phenolic resin. The advantages of using a phenolic resin for this purpose reside in that the molds and cores after curing of the coated resin by a baking procedure are high in physical strength and accordingly convenient for handling and that the baking procedure can be completed in a short time owing to high rate of curing of this resin. In practice, however, the use of a phenolic resin has offered some problems to sand mold processes. First, ammonia gas is liberated during the baking procedure to form the molds and cores as an obstruction to the environmental hygiene because of partial decomposition of hexamethylenetetramine used as a cross-linking agent for the phenolic resin. Second, in aluminum alloy casting processes that are characterized by very low pouring temperatures compared with iron casting processes, the molds and cores remain in a very strong and tough state even at the shake-out stage after solidification of the poured molten metal mainly because of partial carbonization of the phenolic resin, so that the shake-out operation encounters difficulty. Particularly, disintegration of the sand cores needs to be preceded by a firing process in which the mold assemblies containing aluminum alloy casting and sand cores are heated at about 500.degree. C. to thereby decompose the carbonized phenolic resin.
In view of such problems inherent to phenolic resin coated foundry sand, U.S. Pat. No. 4,246,165 proposes to use a binder composition of which principal component is an unsaturated polyester instead of phenolic resin. The proposed resin coated sand does not give out an irritating odor when baked to form molds and cores, and, as a more important advantage, not only molds but also cores formed of this resin coated sand and used in an aluminum alloy casting process can easily be disintegrated at the shake-out stage.
U.S. Pat. No. 4,246,165 specifies to use an unsaturated polyester that is a scarcely tacky solid at room temperature with a view to affording the resin coated sand with good fluidity at room temperature. Actually, however, a resin coated sand according to this U.S. patent is not fully satisfactory in its fluidity and exhibits some tackiness even at room temperature because the polyester coating on the sand particles contains a cross-linking agent, such as diallyl phthalate, which is liquid at room temperature. Particularly in the case of forming a sand mold or core having a thin-wall portion, the insufficiency in the fluidity of this resin coated sand becomes a matter for serious consideration because filling of a die cavity to shape the mold or core is liable to remain incomplete or insufficient in a narrow section where the thin-wall portion of the mold or core is shaped.